1. Field of the Invention
The present invention relates to a system and method for determining the quality of an optical material by measuring and analyzing birefringence (e.g., stress-induced birefringence, inherent birefringence) in the optical material (e.g., glass sheet).
2. Description of Related Art
Birefringence is a characteristic of an anisotropic optical material where the index of refraction depends upon the direction of polarization of light that travels through the optical material. For example, polarized light that is oscillating up and down may be bent more when it passes through the optical material than polarized light that is oscillating from side to side. Birefringence can be inherent to the physical structure of the optical material (e.g., quartz crystals) or it can be induced in the optical material (e.g., glass sheet) by physical stress through the photoelastic effect. There are a number of well-known birefringence sensors that can be used to precisely measure the magnitude and orientation of birefringence in an optical material as described in the patent and articles listed below:                R. Oldenbourg et al. “Polarized Light Microscopy”U.S. Pat. No. 5,521,705, May 28, 1996.        R. Oldenbourg et al. “New polarized light microscope with precision universal compensator” J. Microscopy, V. 180, pp. 140–147, 1995.        B. Wang et al. “A new instrument for measuring both the magnitude and angle of low level linear birefringence” Rev. Sci. Instrum., V. 70, pp. 3847–3854, 1999.The contents of these articles and the patent are hereby incorporated by reference herein.        
Corning Inc. has developed a system that uses one of the well-known birefringence sensors to measure stress-induced birefringence along optical axes perpendicular to a plane of a Liquid Crystal Display (LCD) glass sheet. These stress-induced birefringence measurements are used to determine the stress levels internal to the glass sheet which are indicators of the quality of the glass sheet. To perform an accurate analysis of the stress levels in the glass sheet, multiple discrete birefringence measurements are required, either along the perimeter of the glass sheet or over the area of the glass sheet. And to obtain each discrete birefringence measurement, the system first moves a birefringence sensor to a data point on the glass sheet. The system then lets the birefringence sensor dwell at that data point while the sensor is cycled through multiple optical states and makes multiple power transmission measurements that enable a single birefringence value to be calculated at the data point. After determining the birefringence value at that data point, the system moves the birefringence sensor to the next data point on the glass sheet. The system then lets the birefringence sensor dwell while the sensor is cycled through multiple optical states and makes multiple power transmission measurements that enable a single birefringence value to be calculated at the data point. This process is repeated at each data point on the glass sheet.
The traditional system has a drawback in that it takes a relatively long time to perform the multiple power transmission measurements at one data point on the glass sheet which are needed to calculate a birefringence value. And, as can be appreciated the overall number of birefringence values measured and the total measurement time are at odds with one another since a large number of birefringence measurements provides for better spatial resolution while the total measurement time increases proportionally with the number of birefringence measurements. Another traditional system that has been used to increase the spatial resolution of birefringence measurements without incurring a time penalty includes the use of beam expanding optics to expand the optical measurement beam emitted from the birefringence sensor to illuminate a larger area on the glass sheet, and to use a pixilated detector such as a charge-coupled device (CCD) array. The sensitivity of this system is limited since the CCD array has a small dynamic range and the beam expanding optics introduce polarization impairments. Although the two systems mentioned above successfully enable one to determine the quality of an optical material by measuring and analyzing stress-induced birefringence in the glass sheet, it would be desirable to provide an alternative system that addresses the aforementioned shortcomings and other shortcomings of the traditional systems. This need and other needs are provided by the system and method of the present invention.